Spectroscopy SV nearing completion

Summary:

We are nearly complete with the Science Verification (SV) of the
near-infrared spectroscopy mode of Altair. During the three nights of
June 8-10 (UT), we performed some engineering work and took the first
observations using this mode. A few minor problems appeared
concerning the tradeoff between spectral line profile and image
quality, as well as a need for an improved flexure compensation model
as detailed below. These problems are fairly minor and overall, the
verification of the f/32 spectroscopic mode of NIRI+Altair appears
successful. Periods during the night without SV observations were
used for observations of the NIRI/Altair imaging queue and NIRI only
queue.

Details:

The first half of the first night, June 8, 2004 (UT) was spent
doing some engineering tests before any SV observations:

Tested guider timeout that occurred during the Classical run,
fix required from Victoria

Tested flexure compensation for long spectroscopy sequences

The flexure model proved to be inadequate and we realized that the
original flexure compensation algorithm inside Altair was made for
NIRI in the up-looking port, so a new model needs to be constructed
for the side looking port with help from HIA. Additional flexure data
were obtained during the daytime for a complete model to be
implemented. Data on one SV program was collected on the night of
June 8. Flexure issues were not terrible, but 30 minutes was about
the maximum length of time before objects start drifting out of the
slit. NIRI queue was done for the remainder of the night. Changes to
nssetup were implemented in the daytime for spectroscopic acquisition
on the next nights.

The remaining two nights, June 9 and 10, 2004 (UT) were mostly
filled with SV Altair spectroscopy. Several non-sidereal targets were
observed as well as sidereal targets. Different observing modes were
also tried including one source far from the guide star, a few sources
requiring spectroscopic mapping and one binary source (see images
below). Typically, a centering check was required every 30 minutes or
so while taking spectra, although this was somewhat dependent on the
sky position and the specific program (as some programs require more
strict slit placement than others). Performance seemed in line with
expectations including the resolution of narrowly separated
components.

One issue that was noted early on is the use of the order-sorting
filter. This is a particularly important issue for the H band. If
the H-order sorting filter is used for f/32 Altair spectroscopy, the
image quality along the slit suffers because with the H-order sorting
filter, the NIRI internal focus is set for maximum line profile
quality (ideal for high-resolution spectroscopy). If the H filter is
used rather than the H-order sorting filter, image quality is
maximized at the expense of line profile shape (ideal for
low-resolution spectroscopy). The extent of the image versus
line-profile tradeoff and what to do about is going to be further
examined during daytime. In the mean time, it is probably best to use
the H filter for low-resolution spectra and the H-order sorting filter
for high-resolution spectra. In J and K band observations, the
sorting filters work well and should be used normally.

Some images:

Above: Spectroscopy of a binary source. The primary and secondary are
separated by about 0.4", with about 3 magnitudes of flux difference.
The instrument alignment angle was rotated to put the slit across both
the primary and the secondary and the spectra were easily resolved.
This screenshot is from a 30 second exposure in K-band using the
K-order sorting filter.

Above: First image: a marginally resolved object. This object is
known to be about 155 mas across with an elongated appearance.
Convolving 155 mas with the ~65 mas point spread function yields an
apparent expected size of 168 mas. The full width at half maximum of
the point spread function is outlined in red, and is about 131 mas x
175 mas (6x8 pixels), consistent with expectation. Second image: the
slit has been placed across the short axis of the object. Note that
there is some scattered light in the slit image. We believe this is
due to light scattering off the inside surface of the slit, which is
an uncoated metallic material (matte coating was applied to slitmask
before slit cutting).